1. Field of the Invention
The present invention relates broadly to methods and apparatus for investigating subsurface earth formations. More particularly, this invention relates to borehole tool and methods which utilize flexural waves or their low and high frequency equivalents for measuring formation parameters from which stress induced anisotropy in the formation can be identified.
2. State of the Art
Detailed knowledge of geological formation stresses is desirable in the oil production business, as formation stress determination can affect the planning of stimulation treatments for enhanced oil recovery, as well as provide predictions of sanding and well-bore stability. As a result, there is a growing demand in the art for in situ estimations or determinations of formation stresses. In fact, such in situ determinations can also prove useful with respect to determining the long term stability of the formation and thus the suitability of the formation as a repository for waste disposal.
The stress state of a formation can be completely characterized by the magnitudes and directions of three principal stresses. Generally, the overburden pressure at a given depth in the formation yields the prinicipal stress in the vertical direction. Consequently, identification of the two principal stresses in the horizontal plane is of practical importance.
To date, it has not been possible to make in situ measurements of the stresses in a formation based on acoustic measurements. What has been possible, as suggested by U.S. Pat. No. 4,794,572 to Sondergeld et al. is to use an acoustic borehole tool using a dipole source such as is found in the DSI (dipole shear imager) tool of Schlumberger (see U.S. Pat. Nos. 4,862,991 and 4,850,450 to Hoyle et al., and A. R. Harrison, et al. SPE 20557, pp. 267-282, 1990) to make measurements of formation anisotropy. According to that invention, the tool with the dipole source is used in its normal relatively low frequency mode (e.g., having a spectrum located somewhere between 1 and 4 KHz) to measure formation anisotropy by measuring velocity of two orthogonal dipole modes. As is accepted in the art, the velocities of the two dipole modes measured by the tool are sensitive to the formation anisotropy (including far-field formation stress), with the fast dipole mode directed along the direction of maximum stress, and the slow dipole mode orthogonal to the fast dipole mode. However, formation anisotropy can stem from several sources, including: instrinsic anisotropy due to microstructures such as is often found in shales; formation lithology such as in layered or dipping beds; aligned fractures; and stress induced anisotropy, such as might be due to uniaxial or biaxial stresses in the formation which result from plate tectonic forces. With the techniques of the prior art, it is not possible to distinguish among the-various sources of formation anisotropy, and in particular to distinguish the stress induced anisotropy from the other sources of formation anisotropy. Thus, even though a measurement of formation anisotropy is suggested by U.S. Pat. No. 4,794,572, that measurement has not been usable to measure formation stress.